1. Field of the Invention
The present invention relates to a motor control apparatus. In particular, this invention relates to a motor control apparatus that uses two sets of power supplies.
2. Description of the Related Art
As the power of the elements in computers becomes higher, more heat is generated for example by the central processing unit (CPU). As the quantity of transistors in the CPU has increased from several hundred thousand to five ten million in the Pentium 4, the temperature at the surface of the CPU reaches 50˜80° C. and the temperature of the interior of the CPU is over 80° C. Therefore, a lot of cooling apparatuses are developed for CPUs. Air-cooling is the most popular method. A cooling fan is the key component of the air-cooling apparatus.
The fans can be divided into two kinds, including a direct current (DC) fan and an alternating current (AC) fan. Most of the computers use DC fans, and the DC fans include brushless DC motors. Reference is made to FIG. 1, which shows a block diagram of the fan control apparatus of the prior art. The fan control apparatus includes a hall integrated circuit (IC) 10 and a fan motor 12. The hall IC generates a magnetic change to rotate the fan motor 12. The rotation speed of the fan motor 12 depends on the operation voltage.
In the fan control apparatus of the prior art, the fan motor 12 and the hall IC 10 use the same power supply VCC. However, because the required operation voltage of the hall IC 10 is higher than the starting voltage and the operation voltage of the fan motor 10, the starting voltage and the operation voltage of the fan motor 10 will be affected by the operation voltage of the hall IC 10. When the fan motor 12 is turned on, the rotation speed is too high due to the operation voltage of the hall IC 10 and a heavy noise is generated. Furthermore, when the user wishes to adjust the rotation speed of the fan motor 12, the rotation speed is also affected by the operation voltage of the hall IC 10.
The fan motor 12 needs to cooperate with the control of the hall IC 10 to be turned on and operated. However, due to the operation voltage of the hall IC 10, the starting voltage and the operation voltage of the fan motor 12 is limited. The starting voltage cannot be lowered and the rotation speed cannot be flexibly adjusted. Therefore, the fan control apparatus 1 of the prior art has the following drawbacks. (1) The starting voltage is too high so that a heavy noise is generated. (2) The operation voltage is limited so that the rotation speed cannot be flexibly adjusted.